To accurately describe the geometrical characteristic of shrinkage crack and determine the influence on chloride ion permeability in concrete, by taking samples of shrinkage cracking concrete slab and observing the morphology, the characteristics of shrinkage cracks was statistically analyzed, and the effect law of cracks on chloride ion permeability, tested with ASTM method, was studied. The experimental result shows that the depth and width of shrinkage cracks in concrete are obviously related, which the depth-to-width ratio is 44.31, and the crack tip angle is 2.6 by average. Based on the comparative analysis of the relationship between width parameters e.g. maximum width, minimum width, average width, specific surface area etc. and 6 h electric flux, the maximum width is a suitable parameter for describing the effect of shrinkage crack on permeability of concrete. The permeability coefficient of cracked concrete has a positive linear relation with cracks' maximum width, and the impact of cracks on the permeability coefficient can be neglected when the width is less than 0.18 mm. The width changes of shrinkage cracks significantly affect the impermeability of the concrete, in practical project the allowed maximum width should be determined in accordance with the thickness of concrete cover.